Megasonic cleaning methods and apparatus

ABSTRACT

A cleaning method for use in fabrication of integrated circuit devices includes providing a surface of a substrate assembly. The surface is exposed to a cleaning solution having megasonic energy projected therethrough. Gas bubbles are passed across the surface. A gas flow is introduced into the cleaning solution at a position relative to the surface such that the bubbles formed pass across the surface of the substrate assembly as the bubbles rise in the solution. Further, the pH of the cleaning solution may be controlled by the introduction of the gas bubbles in the cleaning solution. A megasonic cleaning apparatus for carrying out the method is also provided which includes a tank for holding the cleaning solution into which the surface is immersed. A megasonic transducer projects megasonic energy therethrough and a gas feed device provides a gas flow to a position relative to the surface immersed in the tank such that gas bubbles formed in the cleaning solution pass across the surface as the gas bubbles rise in the cleaning solution.

FIELD OF THE INVENTION

The present invention pertains to the cleaning of surfaces in thefabrication of integrated circuit devices. More particularly, thepresent invention relates to megasonic cleaning methods and apparatusfor cleaning surfaces through the use of gas bubbles.

BACKGROUND OF THE INVENTION

Various methods are currently utilized for the removal of smallparticles from surfaces, such as a surface of a wafer in the fabricationof integrated circuit devices. Particles or contaminates are removed bya variety of mechanisms including ultrasonic techniques, high pressurespraying techniques, mechanical scrubbing techniques, etc. Such particleor contaminant removal is commonly carried out after a variety ofprocess steps and before carrying out other process steps. For example,particle or contaminant removal, i.e., cleaning, is necessary after theperformance of sawing, lapping, planarization, polishing, and beforeother device fabrication steps are performed, such as metalization,chemical vapor deposition, epitaxy, etc. Further, and more specifically,for example, after planarization has been performed, such as with theuse of a slurry including silica or alumina particles, a postplanarization clean is performed prior to carrying out other fabricationsteps. The small particles or contaminants resulting from thefabrication steps above, tenaciously hold to a surface and typicallyrequire relatively large forces to remove them, in part due to theelectrostatic potential of such particles.

The use of ultrasonic energy to enhance the cleaning action of solutionsused to clean semiconductor wafer surfaces is well known. Suchultrasonic agitation provided by the ultrasonic energy typically isprovided by using transducers operating at intermediate frequencies,i.e., 20-50 KHz. With the use of ultrasonic energy, bubbles are formedin the cleaning solution in which the wafers are immersed and thebubbles collapse under the pressure of the ultrasonic agitation. Thisproduces shock waves which impinge on the wafer surfaces. The bubblecollapsing is known as cavitation. The shock waves dislodge and displaceparticles to be carried away in the cleaning solution.

Further, the use of high frequency, or megasonic frequencies, forexample, in the range of 0.2-5.0 MHz, is also well-known. The use ofsuch high frequencies has also resulted in improved cleaning,particularly on substrates or wafers with very small, micron sizeelements disposed thereon. Further, use of such high frequenciesprovides a gentler cleaning action on the wafers and/or substrates thanis attainable with an intermediate frequency. Therefore, such gentlercleaning action results in less damage during cleaning operations.

Use of megasonic energy is more efficient than ultrasonic cleaning forsubmicrometer particle removal because it functions via a differentmechanism than ultrasonic cavitation. Because megasonic energy occurs athigher frequencies than ultrasonic energy, the pressure wave that formsgenerates a pulse so rapidly that the vacuum bubbles do not have time toform as in the use of ultrasonic energy. Consequently, megasonic energyconsists of a series of pressure waves. When applied parallel to asurface, these waves dislodge particles. Usually this occurs by allowinga thin film of the cleaning solution, such as deionized water, to formbetween the particle and the surface being cleaned, thereby reducing theattraction between the surface and the particle to facilitate removal ofthe particle.

One illustration of a current method utilized for cleaning wafersurfaces, such as after a planarization process is performed, includesthe immersion of the wafers in deionized water while megasonic energy isprojected therethrough. During the cleaning of the wafers, the tank inwhich the liquid is held, is dumped and refilled one or more times. Thisdumping and refilling appears to draw in air and create an air/liquidinterface across the surface of the wafer which appears to enhance themegasonic cleaning method. However, dumping and refilling the megasonicbath consumes a large amount of the deionized water and/or whateverchemicals might be alternatively utilized as the liquid for performingthe cleaning of the wafer. Further, performing such dumping andrefilling operations is also an inefficient use of time.

Also, in performing cleaning operations, it is in some circumstances,desirable to control the pH of the liquid held by the cleaning tank inwhich the wafers are immersed. For example, after a highly acidic orbasic clean has been performed, a deionized water clean is thentypically used before proceeding with any further process steps.However, it may not be desirable to perform a neutral deionized waterclean immediately following a highly acidic or basic clean. Rather, itmay be more desirable to use an acidic or basic deionized water bath.

Accordingly, there is need in the art for alternative megasonic cleaningmethods which overcome the disadvantages as described above. The presentinvention overcomes these problems and overcomes other problems as willbecome apparent to one skilled in the art from the description below.

SUMMARY OF THE INVENTION

A cleaning method in accordance with the present invention for use infabrication of integrated circuit devices includes providing a surfaceof a substrate assembly. The surface is exposed to a cleaning solutionhaving megasonic energy projected therethrough. The method furtherincludes passing gas bubbles across the surface.

In one embodiment of the method, the passing step includes introducing agas flow at a position relative to the surface of the substrate assemblysuch that a plurality of bubbles are formed. The bubbles pass across thesurface of the substrate assembly as the bubbles rise to a surface ofthe cleaning solution open to an atmosphere.

In another embodiment of the method, the gas may be nonreactive with thecleaning solution or may be reactive with the cleaning solution in orderto alter the pH of the cleaning solution.

In another cleaning method in accordance with the present invention, themethod includes providing a surface of a substrate assembly andimmersing the surface in a cleaning solution having megasonic energyprojected therethrough. The pH of the cleaning solution is controlled byintroducing gas bubbles in proximity to the surface. The gas bubblesinclude a reactive component that modifies the pH of the cleaningsolution.

In yet another method in accordance with the present invention for usein fabrication of integrated circuit devices, the method includesimmersing a surface of a substrate assembly into a cleaning solutionhaving a first pH. The first pH of the cleaning solution is controlledby introducing gas bubbles into the cleaning solution. The gas bubblesinclude a reactive component that alters the first pH of the cleaningsolution to attain a second pH of the cleaning solution that isdifferent than the first pH.

In several embodiments of this method, the surface is previouslysubjected to a cleaning process utilizing a highly acidic or basiccomponent prior to immersing the surface into an acidic or basicdeionized water clean.

A megasonic cleaning apparatus for use in cleaning a surface in thefabrication of integrated circuit devices is also described. Theapparatus includes a tank for holding a cleaning solution into which thesurface is immersed. A megasonic transducer projects megasonic energythrough the cleaning solution. The apparatus further includes a gas feeddevice for providing a gas to a position relative to the surfaceimmersed in the tank such that gas bubbles formed in the cleaningsolution pass across the surface as the gas bubbles rise in the cleaningsolution held in the tank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway perspective view of a megasonic cleaning apparatusin accordance with the present invention.

FIG. 2 is an illustrative side view of a portion of the megasoniccleaning apparatus of FIG. 1 with one side wall removed.

FIG. 3 is an illustrative end view of a portion of the megasoniccleaning apparatus of FIG. 1 with one end wall removed.

FIG. 4 is a detailed view of a portion of a wafer immersed in themegasonic cleaning apparatus of FIG. 1.

FIGS. 5 and 6 are flow diagrams of illustrative methods for using thegas bubbles in accordance with the present invention for in situ controlof the pH of the cleaning solution.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, reference is made to theaccompanying figures which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention. It is to beunderstood that other embodiments may be utilized and that changes maybe made without departing from the scope of the present invention asdefined in the accompanying claims.

The terms wafer and substrate assembly are used interchangably hereinand include any semiconductor based structure, i.e., such as asemiconductor substrate having one or more layers or structures formedthereon. Both terms are to be understood as includingsilicone-on-sapphire (SOS) technology, silicon-on-insulator (SOI)technology, doped and undoped semiconductors, epitaxial layers ofsilicon supported by a base semiconductor, as well as any othersemiconductor based structures well known to one skilled in the art.Furthermore, when a reference is made to a wafer or a substrate assemblyin the following description, previous process steps may have beenutilized to form regions/junctions in a semiconductor based structurepreviously formed. The following detailed description is, therefore, notto be taken in a limiting sense, as the scope of the present inventionis defined by the appended claims. The present invention contemplatesthe cleaning of any surface of a wafer, semiconductor substrate orsubstrate assembly and is in no manner limited to any particularsurface.

The megasonic cleaning apparatus 10 in accordance with the presentinvention shall be described with reference to FIGS. 1-4. Thereafter,illustrative methods of cleaning particles from a surface using methodsin accordance with the present invention shall be described withreference to FIGS. 1-6. As shown in FIG. 1, megasonic cleaning apparatus10 includes a tank housing 12, such as a quartz tank housing, which issuitable to hold a cleaning solution 13 therein. The cleaning solution13 may include any known liquid for accomplishing a desired cleaning oretching of one or more surfaces of one or more semiconductor substrates,substrate assemblies, or wafers, such as shown generally by wafers 11 inFIG. 1. For example, the cleaning solution 13 may be deionized water, anRCA solution, a diluted HF solution, an ammonium hydroxide solution, anyother weak or strong acid, or any other weak or strong base.

The wafers 11 are typically supported within the cleaning solution 13within the tank housing 12 by a holder or a rack (not shown) for ease ofimmersion into the cleaning solution 13. Such a holder may include amechanical agitation device for agitating the holder within the cleaningsolution 13 to enhance cleaning in the megasonic cleaning apparatus 10.The holder may be moved under robotic control as is known to one skilledin the art. However, any holder and movement mechanism for immersing thewafers 11 in the cleaning solution 13 is suitable. Further, any numberof wafers, substrate assemblies, etc. may be immersed for cleaning inthe cleaning solution 13.

The tank housing 12 includes a first sidewall 14 and a second sidewall16 extending substantially orthogonally from respective sides of abottom wall 22. Further, the tank housing 12 includes end walls 18 and20 extending substantially orthogonally to respective ends of bottomwall 22 to define a volume within the tank housing 12 in which thecleaning solution 13 is held. The tank housing 12 includes an input port23 for introduction of cleaning solution 13 therein and may bepositioned at any suitable location. For example, the input port 23 maybe an opening in the tank housing fitted with an input feed line asshown in FIG. 1, or may be any other method of introducing the cleaningsolution 13 into the tank housing 12, such as a line running down one ofthe walls of the tank housing 12.

The tank housing 12 also includes an overflow region for the overflow ofcleaning solution 13 having particles captured thereby cleaned from thesurfaces of the wafers 11. Such particles typically rise to the surface17 of the cleaning solution 13 and are transported out of the tankhousing 12. The overflow region shown in FIG. 1 is the entire perimeterof side and end walls defining the top opening into the tank housing 12such that cleaning solution having removed particles therein may betransported or overflowed out of the tank housing 12 at any point alongthe perimeter defining the opening. However, one skilled in the art willrecognize that any overflow mechanism may be used, such as, for example,a lower portion defined in one of the walls, an opening in several wallsor any other suitable method for transporting particles in the cleaningsolution out of the tank housing 12.

Further, a recirculation apparatus (not shown) may be used inconjunction with the present invention to collect overflow from the tankhousing 12, filter such overflow to remove particle or contaminants andthen recharging the filtered cleaning solution back into the tankhousing 12, such as, via input port 23. For example, if the cleaningsolution is deionized water, a recirculation apparatus is typically notused. However if the cleaning solution is more difficult to dispose ofor costly to use, then recirculation may be utilized.

The megasonic cleaning apparatus 10 includes one or more megasonictransducers 24, such as an array of transducers, for projectingmegasonic energy through the cleaning solution 13 held in the tankhousing 12. The one and more megasonic transducers 24 produce waves ofessentially single and constant frequency, which results in asubstantially uniform pattern of standing waves being set up within thecleaning solution 13 as represented by arrows 26. The standing wavestravel parallel to the surface to be cleaned. Further, the standingwaves 26 are reflected off end wall 18 and thus, to reduce thereflection of the waves that may cause destructive interference with thestanding waves projected through the cleaning solution 13 by one or moretransducers 24, a quartz deflector plate 19 is utilized to deflect thestanding waves as shown in FIG. 1. Such a destructive interferenceproblem may be corrected in this or any other number of manners, andcorrective measures are in no manner critical to the present inventionas described in the accompanying claims.

The megasonic transducers 24 may include any suitable megasonictransducer for projecting megasonic energy through cleaning solution 13.For example, the suitable transducers project energy at high frequenciesin the range of about 0.2 to about 5.0 MHz. The megasonic transducer maybe positioned at any of the walls forming the tank housing 12, includingany side wall, end wall or bottom wall, as long as the waves projectedthrough the cleaning solution 13 are parallel to the surface of thewafers to be cleaned. Even transducers positioned to project energy downinto the solution from the top of the apparatus may be utilized.

For example, such transducers 24 may include transducers as utilized inmegasonic cleaning systems such as the SMS Semi-Automatic Wet Systemincluding megasonic cleaning available from SubMicron Systems, Inc., ora megasonic wafer cleaning system available from Verteq Inc., STEAG, orSanta Clara Plastics. Further, each of these systems or any othermegasonic cleaning system may be modified to practice the presentinvention as described in accompanying claims. The control of thetransducers 24, as well as any other controllable element of thecleaning apparatus, for example, a robotic holder, is shown generally inFIG. 1 as apparatus controller 27.

As described in the Background of the Invention section, the megasonicenergy consists of a series of pressure waves 26. When the waves 26 areapplied parallel to surfaces 15 of wafers 11 as shown in FIGS. 1-4, thewaves dislodge particles. This is accomplished by allowing a thin filmof the cleaning solution 13 to form between the particles and a surface15, thereby reducing the attraction between the surface 15 and theparticles to facilitate removal thereof.

In accordance with the present invention, in order to enhance theability of the megasonic energy to remove the particles from thesurfaces 15 of the wafers 11, gas bubbles 36 are passed across thesurfaces 15. Gas bubbles passing across a surface refers to the bubblespassing at least in close proximity to surfaces and further may includehaving some of the gas bubbles being in contact with surfaces 15 (FIG.4).

To facilitate the passing of gas bubbles 36 across the surfaces 15, agas is provided, such as, for example, by injection, into the cleaningsolution 13 at a position relative to the wafers 11 such that movementof the bubbles in the solution is across the surfaces 15. Preferably,the gas bubbles 36 are provided at a position below the wafers 11 andbetween the wafers 11 and bottom wall 22 of the tank housing 12. Forexample, as generally shown in FIGS. 1-3, a gas feed line 34 is providedwhich is terminated within the tank housing 12 by a gas injection device30. Although the gas feed line 34 is shown as being positioned along anddown a wall of the tank housing 12, it should be apparent that anymanner of getting the gas to the desired position is suitable. Forexample, a gas feed line through the bottom wall, an end wall, or anyfeeding mechanism may be suitable.

Any gas injection device 30 may be utilized which is suitable forproducing gas bubbles 36 at a position relative to the wafers 11 suchthat when the gas bubbles move towards the surface 17 of cleaningsolution 13 which is open to the atmosphere, the gas bubbles 36 passacross the surfaces 15 of wafers 11. Such gas bubbles 36 passing acrossthe surfaces 15 enhance the megasonic cleaning performed by theapparatus 10.

For example, in the generalized illustrative embodiment of FIG. 1, thegas injection device 30 may be an integral portion of the gas feed line34 itself having one or more openings 31 at the terminated end of thefeed line 34. In addition, the injection device may be one or more gasfeed lines terminated below the wafers having one or more openingsdefine therein. For example, the openings may be a quarter of an inchapart defined in the gas feed line along the entire or only a portion ofthe bottom wall of the tank housing 12. Further, the injection device30, may be a gas manifold having a plurality of openings defined thereinfor producing a substantial number of gas bubbles in the cleaningsolution 13. Likewise, the openings for injecting bubbles into thesolution held in the tank housing 12, may be defined in the bottom wall22 as opposed to a separate structure within the tank housing 12. Forexample, multiple gas feed lines provided to multiple openings in thebottom wall 22 may be used.

In another embodiment of the injection device 30, the device 30 mayinclude a sparging apparatus for receiving gas through the gas feed line34 and which provides a cloud of very small bubbles. A spargingapparatus may include any device suitable for providing such a cloud ofsmall bubbles. For example, the sparging apparatus may make use of abaffle structure, a spraying device, or any other technique. The smallbubbles created by the sparging apparatus are much like those createdusing known apparatus, such as commonly known and used aeration devices.

It should be readily apparent that any injection device suitable forproviding gas bubbles to a desired position in the cleaning solution maybe utilized in accordance with the present invention as shown by thenumerous and various configurations described above. In each of theabove described embodiments, the openings through which a gas isprovided into the cleaning solution 13 are described as being providedto a position below the wafers 15 and between the wafers 15 and thebottom wall 22. It is possible that gas bubbles could be providedthrough some other mechanism to an alternative position such that theypass across the wafer surfaces 15, i.e., for example, such as betweenthe wafers 11 and a side or end wall. However, it is preferable, thatthe gas bubbles 36 are provided below the wafers 11 so as to naturallyrise up and across the surfaces 15 of the wafers 11 to the surface 17 ofcleaning solution 13 open to the atmosphere carrying with them particlescleaned from surfaces 15.

The enhanced cleaning action of the megasonic clean appears to be due toone of several factors. The gas bubbles 36 which pass across thesurfaces 15 of wafers 11 form gas/liquid/surface interfaces 37 at thesurfaces 15 of wafers 11. This interface 37 is believed to form areflection point for the megasonic energy which in addition to thesurface tension between the gas, liquid and surface enhances particleremoval and transport away from the wafer surfaces 15. Any number ofbubbles formed which pass across the surfaces 15 provides benefit.However, by using a sparging apparatus as the injection device 30, it isbelieved that as the small cloud of bubbles passes across the wafers 11,there is a further increase in the amount of liquid/gas/surfaceinterface 37 increasing the efficiency of the megasonic cleaning usingthe megasonic cleaning apparatus 10.

Further, the enhanced cleaning action is also believed to be due to theuse of the gas bubbles to carry particles which have been dislodged bythe megasonic clean away from the surfaces 15 of the wafers 11 beforethey redeposit thereon. Such transport to the surface 17 of the cleaningsolution 13 is accomplished by the natural rise of the bubbles 36.

The gas injected into the cleaning solution 13 to form bubbles 36 may beany gas which is nonreactive with the cleaning solution 13. For example,such gases may include nitrogen, argon, helium, or any other gas that isnonreactive with the cleaning solution 13. In one illustrativeembodiment, the cleaning solution 13 may be deionized water and the gasinjected therein may be one of the above listed nonreactive gases.

In another embodiment, gases which react with cleaning solution 13 mayalso be utilized so as to enhance the clean through bubbles passingacross the surfaces 15 and/or provide an effective means of controllingor altering the pH of the cleaning solution 13 utilized in the megasoniccleaning apparatus 10. For example, a gas having a reactive componentcan be injected into the cleaning solution 13 which alters the pH of thecleaning solution 13. The gas injected may include a single gascomponent, such as carbon dioxide, or may be a gas mixture, such as anonreactive gas including a reactive component, such as argon containinga small amount of ammonia. The amount of gas may be controlled by one ormore flow controllers, mixers, or any other devices utilized forsupplying gases as known to one skilled in the art, shown generally asgas flow controller 29 in FIG. 1.

Several illustrations of the use of gas bubbles to control the pH of acleaning solution are described with reference to FIGS. 1-6, and moreparticularly, with reference to FIGS. 5 and 6. Generally, in a firstmethod 70 as shown in FIG. 5, a cleaning solution is provided which hasa first pH as generally represented by block 72. The pH of the cleaningsolution provided (block 72) can be attained by providing gas bubblesthereto in a manner and to a position in the cleaning solution asdescribed above or in any other known manner and/or at any positionwithin the tank housing holding the solution. For example, if adeionized water bath is performed and the desired pH is to be acidic,then a control flow of carbon dioxide may be introduced into thedeionized water in the tank housing 12 as bubbles to form some carbonicacid therein. Alternatively, a cleaning solution already having thefirst pH may be introduced into the tank 12. The pH of the cleaningsolution may be acidic, basic or may be neutral, as in a neutraldeionized water bath.

Introduction of gas bubbles into the tank housing 12 to control the pHof the cleaning solution can effectively control pH as compared to othermethods of adjusting the pH. For example, introducing gas bubbles intothe tank 12 by a gas feed line has a reduced residual effect as comparedto the use of trying to control the pH with a solution. In other words,the residual effect of gas remaining in a gas feed line is somewhatreduced as compared to the effect of residual solution remaining in aline providing a liquid pH altering component to the cleaning solution.Further, the dumping and refilling of the tank to provide a cleaningsolution having a different pH is eliminated, i.e, such as in the use ofspiked tanks. Further, introduction of gas bubbles into the tank isgenerally simpler than other methods of spiking the tank, i.e., liquidsolution spiking. The storage of the gas for production of gas bubblesis generally safer, i.e., storage of carbon dioxide to form carbonicacid by gas bubbling is simpler and safer than storage and use ofcarbonic acid for liquid spiking. In addition, the likelihood ofoverspiking is reduced when using a gas bubbling technique that can bemore readily controlled.

With the cleaning solution 13 having a first pH provided in the tank 12,one or more surfaces, i.e., wafers 11, to be cleaned are immersed in thesolution 13 as represented by block 73. Thereafter, the clean isperformed, and the pH of the cleaning solution may be altered to attaina second pH by introducing gas bubbles into the solution 13 (block 74).The pH may be altered either from neutral to basic, basic to neutral,acidic to neutral, neutral to acidic, or basic to acidic, basic to lessor more basic, acidic to less or more acidic, or any combination thereofor along any continuum therebetween.

Examples of such pH modification, includes injection of carbon dioxideinto a neutral deionized water cleaning solution. As such, carbonic acidis formed in the deionized water to alter the pH of the cleaningsolution 13 into the acidic regime. As opposed to altering the pH intothe acidic region, a gas mixture of a nonreactive gas containing a smallpercentage of ammonia which will solubilize in the deionized watermaking the pH of the cleaning solution more basic can be utilized.

As shown, different pH's of the cleaning solution 13 can be attained insitu while the cleaning solution is in the tank housing 12 before orafter the wafers are immersed in the solution. Therefore, such in situalteration of the cleaning solution pH is possible in conjunction withenhancement of the megasonic cleaning of the wafers by passing bubblesacross the wafers as megasonic energy is projected through the cleaningsolution without dumping and refilling the cleaning solution 13, i.e.,deionized water or any other chemical solution. Alternatively, the pHmay be controlled separately, i.e., before the wafers are immersed.

The illustrations using ammonia and carbon dioxide above with regard tocontrolling the pH of the cleaning solution, i.e., deionized water, areclearly not an exhaustive list of the cleaning solutions or gases whichmay be utilized in accordance with the present invention to controlparameters of the cleaning solution in situ during the cleaning process.For example, NH₃ may be used to control the pH of an RCA clean, CO₂ andCl₂ may be used to control the pH of an acidic clean, SF₆ may be used tocontrol the pH of an HF clean, etc. Generally, the pH of any cleaningsolution can be modified in a manner as described herein depending uponthe application with which the clean is used, for example, afterplanarization of a metalization step.

Illustration of the control of pH and advantages of such control aredescribed with reference to FIG. 6 and with respect to deionized watercleans after planarization of surfaces, such as, for example, surfacesincluding metalized regions and/or silicon based material regions, i.e.,BPSG. For example, planarization or polishing of a surface havingmetalized regions, with for example, a slurry including alumina orsilica particles, or such particles fixed on a pad, or any otherplanarization technique, result in silica or alumina particles and/orother ionic contaminants which need to be cleaned from the surface.Typically, after metal planarization is performed, a highly acidic cleanis used, such as an HF clean, i.e., vapor or dip, as is commonly knownto those skilled in the art. Such a clean typically has a pH less thanabout 2. On the other hand, when performing a planarization of a surfacehaving silicon-based regions, a highly basic clean is typically utilizedsuch as an ammonium clean (i.e., an RCA clean), as is commonly known tothose skilled in the art. Such a basic clean typically has a pH greaterthan about 10.5. Both highly acidic or highly basic cleans arerepresented generally by block 92 of the cleaning method 90 of FIG. 6.

However, such cleans (block 92) do not complete the cleaning process asa deionized water bath is typically necessary prior to the completion ofany further fabrication steps. The deionized water bath is typicallynecessary to stop the previous clean or etch step which utilized ahighly basic or highly acidic component. For example, when cleaning anoxide containing surface, i.e., silicon oxide, BPSG, etc., the deionizedwater bath stops the clean or etch of the surface before undesired oxideremoval occurs. However, it is undesirable to immerse wafers subjectedto either highly basic or highly acidic cleans (block 92) directly intoa neutral deionized water megasonic clean as the effectiveness of such aclean is reduced due to the shock of pH change from one clean to theother. In other words, the neutral nature of the deionized water bath,as opposed to the negative or positive nature of an acidic or basicdeionized water bath, reduces the effectiveness of the clean. In thecase of the neutral deionized water bath, repulsive forces from thecharged nature of the highly acidic or basic cleaning component are lostwhen the surface is immersed in the neutral bath. The neutral bathresults in the possible redeposition of particles back on the surfacebeing cleaned, reducing the effectiveness of the cleaning process. Whena basic or acidic deionized water bath is used, the repulsive forces arestill present, such that the carrying away of particles away from thesurface is enhanced. In other words, for the deionized water clean to bemore effective, the deionized water bath should be acidic, i.e., at a pHin the range of about equal to the pH of the previously used highlyacidic component to about 5, when the previous clean was highly acidic.Further, the bath should be basic, i.e., at a pH in the range of aboutequal to the pH of the previously used highly basic component to about8, when the previously clean was basic. The present invention allows forin situ control of the pH during such a clean process, i.e., alteringthe pH in the same tank housing having the same cleaning solutionwithout dumping and refilling.

It should be apparent that the pH of the deionized water bath iscontinuously altered because as the solution 13 of the bath overflows,additional deionized water is input, such as through port 23. As such,to maintain the pH of the bath as desired, gas bubbles are injected byinjection device 30. Such injection may be in a continuous fashion ornoncontinuous fashion. Further, the gas bubbles may include a reactivecomponent for a partial time period or throughout the entire process,i.e., the use of reactive and nonreactive gas bubbles may be used invarious combinations.

As represented by block 94, prior to immersing the wafers into thedeionized water bath, the pH of the deionized water is altered byproviding gas bubbles in the deionized water. For example, as previouslydescribed, providing carbon dioxide to the solution forms some carbonicacid to take the solution into the acidic region, whereas providing anammonia component to the solution takes the deionized water into thebasic region. Such an introduction of the gas bubbles may be performedin any manner as previously described.

After the wafers are immersed into the basic or acidic deionized waterbath (block 94) (depending on whether the previous clean was acidic orbasic), the pH of the solution can be altered as desired (block 96). Forexample, the pH of the bath is altered in the manners as described abovewith gas bubbles, such as with the use of ammonia or carbon dioxide.

It should be readily apparent that the above cleaning illustrations aregenerally described to show the introduction of gas bubbles intocleaning processes to obtain the advantages of pH control. These processare not the only cleaning processes which may benefit from such pHcontrol and the present invention is not limited to the illustrationsgiven, but only in accordance with the accompanying claims. For example,other cleaning processes which may benefit may include a gate oxideclean, a pre-silicon nitride deposition clean, a post planarizationclean, and any other clean that may benefit from the invention herein,such as other oxide cleans.

Although the present invention has been described with particularreference to various embodiments thereof, variations and modificationsof the present invention can be made within a contemplated scope of thefollowing claims, as is readily known to one skilled in the art.

What is claimed is:
 1. A cleaning method for use in fabrication ofintegrated circuit devices, the method comprising the steps of:providinga surface of a substrate assembly; immersing the surface in a cleaningsolution having megasonic energy projected therethrough; and providinggas bubbles to a position relative to the surface of the substrateassembly such that the gas bubbles pass naturally in the cleaningsolution across the surface.
 2. The method according to claim 1, whereinthe providing step includes introducing a gas flow at a positionrelative to the surface of the substrate assembly such that a pluralityof bubbles are formed, and further wherein the bubbles pass across thesurface of the substrate assembly as the bubbles rise in the cleaningsolution.
 3. The method according to claim 2, wherein the providing stepincludes passing the plurality of bubbles in close proximity to thesurface with a portion of the bubbles contacting the surface.
 4. Themethod according to claim 2, wherein the providing step includesintroducing the gas flow below the surface of the substrate assembly toform the plurality of bubbles, and further wherein the providing stepincludes passing the plurality of bubbles in close proximity to thesurface with a portion of the bubbles contacting the surface.
 5. Themethod according to claim 1, wherein the gas bubbles are formed from agas that is nonreactive with the cleaning solution.
 6. The methodaccording to claim 1, wherein the cleaning solution has a pH, andfurther wherein the gas bubbles are formed from a gas mixture includinga nonreactive component that does not react with the cleaning solutionand a reactive component that reacts with the cleaning solution to alterthe pH of the cleaning solution.
 7. The method according to claim 6,wherein the cleaning solution is deionized water and the reactivecomponent is one of ammonia and carbon dioxide.
 8. The method accordingto claim 1, wherein one of silica or alumina particles are removed fromthe surface, the silica or alumina particles resulting fromplanarization of the surface.
 9. A cleaning method for use infabrication of integrated circuit devices the method comprising thesteps of:providing a surface of a substrate assembly; immersing thesurface in a cleaning solution having megasonic energy projectedtherethrough; introducing a gas flow to a sparging apparatus at aposition relative to the surface of the substrate assembly such that aplurality of bubbles are formed, and further wherein the bubbles passacross the surface of the substrate assembly as the bubbles rise to asurface of the cleaning solution.
 10. A cleaning method for use infabrication of integrated circuit devices, the method comprising thesteps of:providing a surface of a substrate assembly; immersing thesurface in a cleaning solution having megasonic energy projectedtherethrough; and controlling the pH of the cleaning solution byintroducing gas bubbles into the cleaning solution and passing the gasbubbles across and in proximity to the surface, wherein the gas bubblesinclude a reactive component that modifies the pH of the cleaningsolution.
 11. The method according to claim 10, wherein the cleaningsolution has a first pH, and further wherein the controlling stepincludes the step of modifying the first pH of the cleaning solution toa second pH while the surface remains immersed in the cleaning solution.12. The method according to claim 11, wherein the first pH of thecleaning solution is attained by introducing gas bubbles into thecleaning solution prior to immersing the surface into the cleaningsolution.
 13. The method according to claim 10, wherein the gas bubblesare formed from a gas mixture including a nonreactive component thatdoes not react with the cleaning solution and a reactive component thatreacts with the cleaning solution to modify the pH of the cleaningsolution.
 14. The method according to claim 13, wherein the cleaningsolution is deionized water and the reactive component is one of ammoniaand carbon dioxide.
 15. A megasonic cleaning apparatus for use incleaning a surface in the fabrication of integrated circuit devices, theapparatus comprising:a tank for holding a cleaning solution into whichthe surface is immersed; a megasonic transducer for projecting megasonicenergy into the cleaning solution; and a gas feed device for providing agas to a position relative to the surface immersed in the tank such thatgas bubbles formed in the cleaning solution pass across the surface asthe gas bubbles naturally rise in the cleaning solution held in thetank.
 16. The apparatus according to claim 15, wherein the tank furtherincludes a bottom wall, the gas feed device including one or moreopenings for providing the gas to a position between the bottom wall andthe surface.
 17. The apparatus according to claim 15, wherein the gasfeed device includes a sparging apparatus.
 18. The apparatus accordingto claim 15, wherein the apparatus further includes a control apparatusfor controlling the flow of one or more gases to the gas feed device.19. The apparatus according to claim 15, wherein the gas bubbles areformed of a gas that is nonreactive with the cleaning solution.
 20. Theapparatus according to claim 15, wherein the gas bubbles are formed froma gas mixture including a nonreactive component that does not react withthe cleaning solution and a reactive component that reacts with thecleaning solution to alter the pH of the cleaning solution.